A fuel cell is an electric power generation system that converts chemical reaction energy of oxygen and hydrogen contained in a hydrocarbon-based material such as methanol, ethanol, and natural gas directly into electrical energy.
The fuel cell can be classified into a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type of fuel cell depending upon the kind of electrolyte used. Although each fuel cell basically operates in accordance with the same principles, the kind of fuel, the operating temperature, the catalyst, and the electrolyte may be selected depending upon the type of cells.
Recently, a polymer electrolyte membrane fuel cell (PEMFC) in which power characteristics are superior to that of conventional fuel cells, operating temperature is lowered, and starting and response characteristics are quicker, has been developed. It has advantages in that it can be applied to wide fields such as a transportable electrical power source for an automobile, a distributed power such as for a house and a public building, and a small electrical power source for an electronic device.
According to the above-mentioned fuel cell system, the stack substantially generating the electricity has a structure in which several or several tens of unit cells consisting of a membrane electrode assembly (MEA) and a separator (also referred to as “bipolar plate”) are laminated together. The membrane electrode assembly is composed of an anode (also referred to as a “fuel electrode” or an “oxidation electrode”) and a cathode (also referred to as an “air electrode” or a “reduction electrode”) separated by the polymer electrolyte membrane.
The polymer electrolyte membrane for the electrolyte is commercially available as a perfluorosulfonic acid ionomer membrane such as NAFION™ (fabricated by DuPont), FLEMION™ (fabricated by Asahi Glass), ASIPLEX™ (fabricated by Asahi Chemical), and DOW XUS™ (fabricated by Dow Chemical).
Conventional polymer membranes such as NAFION™ have advantages of excellent proton conductivity and high chemical-resistance, and they are not corroded easily. However, they have shortcomings in that they are expensive, and methanol crossover can be generated. Also, since the movement of H+ protons requires water, a humidifier should be additionally included. Therefore, the setup cost is high and a large setup space is required. In addition, when the fuel cell is operated at a high temperature, the moisture is evaporated and thus the proton conductivity is degraded.